This invention presents a set of methods and apparatus for omnidirectional stereo imaging. By xe2x80x9comnidirectional imaging systemxe2x80x9d, we mean a system that is able to acquire images with a field-of-view covering the entire solid angle of a hemisphere (2xcfx80 steradians) without any mechanical moving parts. Consequently, the field of view of an omnidirection imaging system has an in plane view angle of 360 degrees. The field of view of a conventional camera or a light projector can be dramatically increased by employing a reflective mirror properly placed in front of the camera or the projector. A pair of omnidirectional cameras is able to form a unique stereo imaging system that is able to obtain three dimensional images of a surrounding scene with 360 degree view angle. A combination of an omnidirectional camera and an omnidirectional structured light projector cart also provide a means to obtain quantitative three dimensional measurements of the objects around the camera system. The omnidirectional three dimensional imaging methods and apparatus presented herein may offer unique solutions to many practical systems that need simultaneous 360 degree viewing angle and three dimensional measurement capability.
A number of approaches have been proposed in the past for imaging systems to achieve wide field-of-view (FOV). None of them, however, is able to generate 3D omnidirectional images.
Most existing imaging systems employ electronic sensor chips, or still photographic film, to record optical images collected by the imaging system""s optical lens system. The image projection for most camera lenses is modeled as a xe2x80x9cpin-holexe2x80x9d with a single center of projection. The sizes of camera lenses and imaging sensors have practical limitations, such that the light rays that can be collected by a camera lens and received by the imaging device typically form a cone having a very small opening angle. Therefore, angular field-of-views for conventional cameras are within a range of 5 to 50 degrees. For example, an 8.5 mm F/1.3 camera lens for 1/2xe2x80x3 CCD (Charge Coupled Device) chip only has an angular FOV of 41.2 degrees.
Optical engineers have designed several versions of wide-viewing-angle lens system, called fish-eye lenses. See Wood, R.W., Fish-Eye View and Vision Underwater, Philosophical Magazine, 12 (Series 6):159-162, 1906; Miyamoto, K, Fish-Eye Lens, J. Optical Soc. America, 54(8):1060-1061, 1964. Fish-eye lenses feature a very short focal length which, when used in place of conventional camera lenses, enables the camera to view objects over much a wider angle (almost 2xcfx80 steradians). In general, a wider FOV requires a more complicated design for the fish-eye lens. To obtain a hemispherical FOV, the fish-eye lens must be quite large in dimension, complex in optical design, and hence expensive. Also, it is very difficult to design a fish-eye lens that ensures single view point constraint, i.e., all incoming principal light rays intersect at a single point to form a fixed viewpoint. The use of fish-eye lenses for wide FOV imaging application has been advocated. See Oh, S.J, and Hall, E., Guidance of a Mobile Robot Using an Omni-directional Vision Navigation System, Proc. SPIE, 852:288-300, Nov.,1987; U.S. Pat. No. 5,359,363 issued to Kuban, D.P., et al, Oct. 25, 1994.
Although the image acquired by fish-eye lenses may prove to be good enough for some visualization applications, the distortion compensation issue has not been resolved. In addition, the high unit-cost issues remain to be major hurdles for its wide-spread application. The fish-eye lens technique adopts a statically positioned camera to acquire a wide angle of view. However, the nonlinear property resulting from the semi-spherical optical lens mapping makes the resolution along the circular boundary of the image very poor. Further, the field of view corresponding to the circular boundary of the image usually represents a ground or floor where a high resolution image is required.
Large field of view of objects may be obtained by using multiple cameras in the same system, each pointing in a different direction. However issues related to seamless integration of multiple images are further complicated by the fact that each image produced by each camera has different centers of projection. The cost for such a system is usually high. The image processing required by using multiple cameras or by rotating cameras in order to obtain precise information on position and azimuth of an object takes a long time, which is not suitable for real-time battle field modeling and reconnaissance applications.
Another straightforward solution to increasing the FOV is illustrated in FIGS. 1A-1B. An imaging system (100) is rotated about its center of projection (110). An image sequence (120) of individual images (130) is acquired by the imaging system (100) at different positions. The images (130) are xe2x80x9cstitchedxe2x80x9d together to obtain a panoramic view of the scene (140) as seen in FIG. 1B. Such an approach has been recently proposed by several researchers. See Chen,S. E., QuickTime VRxe2x80x94An Image Based Approach to Virtual Environment Navigation, Computer Graphics: Proc. Of SIGGRAPH 95, 29-38, 1995; McMillam, L, and Bishop, G., Plenoptic Modeling: An Image-Based Rendering System, Computer Graphics: Proc. Of SIGGRAPH 95, 38-46, 1995; Zheng, J.Y., and Tsuji,S, Panoramic representation of scene for route understanding, Proc. 10 Int""l Conf. Pattern Recognition, 1:161-167, 1990. A very interesting approach has also been developed that employs a camera with a non-frontal images detector to scan the world. See Krishnan, A, and Ahuja, N., Panoramic Image Acquisition, Proc. Of IEEE Conf. Computer Vision and Pattern Recognition (CVPR-96), 379-384,1996.
The first disadvantage of any rotating image system is that it requires the use of moving parts and precision positioning devices. A more serious drawback is that such systems lack the capability of simultaneously acquiring images with wide FOV. Although such systems can acquire precise azimuth information in omnidirectional view, the imaging process is time-consuming and the method is not applicable to real-time problems such as avoiding collision against moving obstacles or monitoring scene with mobile objects. This restricts the use of rotating systems to static and/or non-real-time applications.
In contrast the present apparatus is capable of capturing real-time omnidirectional images without using any moving parts. By xe2x80x9comnidirectional imagesxe2x80x9d, we mean images with a field-of-view covering an entire hemisphere (2xcfx80 steradians of solid angle) simultaneously. FIGS. 2A-2B provide a comparison between fields of view of the present omnidirectional camera (200) and those of panoramic (210) and conventional (220) cameras as illustrated by the shaded portions (200-220). As one can see, a panoramic camera is not omnidirectional, since it can only provide a wide-angle of FOV at certain time instances. Further, this FOV is not in all directions. On the other hand, the field of view of the omnidirectional camera (200) covers an entire hemisphere or 2xcfx80 steradians of solid angle.
The primary objective of present invention is to provide a set of simple methods and apparatus to obtain simultaneously omnidirectional stereo images without requiring the use of moving parts. Accordingly, an improved imaging apparatus for generating a two dimensional image, includes a substantially hyperbolic reflective mirror configured to satisfy an optical single viewpoint constraint for reflecting a scene, an image sensor responsive to the reflective mirror and that generates two dimensional image data signals, and a controller coupled to the image sensor to control a display of two dimensional object scenes corresponding to the image data signals. The field of view of a conventional camera or a light projector can be dramatically increased by employing a reflective mirror properly placed in front of the camera or the projector. A pair of omnidirectional cameras is able to form a unique stereo imaging system that is able to obtain three dimensional images of surrounding scene with 360 degree view angle. A combination of an omnidirectional camera and an omnidirectional structured light projector can also provide a means to obtain quantitative three dimensional measurements of the objects around the camera system. The omnidirectional three dimensional imaging methods and apparatus presented herein may offer unique solutions to many practical systems that need simultaneous 360 degree viewing angle and three dimensional measurement capability.